The XSetDeviceFocus request changes the focus
of the specified device and its last-focus-change time. It has no effect
if the specified time is earlier than the current last-focus-change time
or is later than the current X server time. Otherwise, the last-focus-change
time is set to the specified time CurrentTime is replaced by the current
X server time). XSetDeviceFocus causes the X server to generate DeviceFocusIn
and DeviceFocusOut events.

Depending on the focus argument, the following
occurs:

If focus is None , all device events are discarded until a new
focus window is set, and the revert_to argument is ignored.

If focus is
a window, it becomes the device's focus window. If a generated device event
would normally be reported to this window or one of its inferiors, the
event is reported as usual. Otherwise, the event is reported relative to
the focus window.

If focus is PointerRoot, the focus window is dynamically
taken to be the root window of whatever screen the pointer is on at each
event from the specified device. In this case, the revert_to argument
is ignored.

If focus is FollowKeyboard, the focus window is dynamically
taken to be the window to which the X keyboard focus is set at each input
event.

The specified focus window must be viewable at the time XSetDeviceFocus
is called, or a BadMatch error results. If the focus window later becomes
not viewable, the X server evaluates the revert_to argument to determine
the new focus window as follows:

If revert_to is RevertToParent, the focus
reverts to the parent (or the closest viewable ancestor), and the new
revert_to value is taken to be RevertToNone.

If revert_to is RevertToPointerRoot,
RevertToFollowKeyboard, or RevertToNone, the focus reverts to PointerRoot,
FollowKeyboard, or None, respectively.

When the focus reverts, the X server
generates DeviceFocusIn and DeviceFocusOut events, but the last-focus-change
time is not affected.

Input extension devices are not required to support
the ability to be focused. Attempting to set the focus of a device that
does not support this request will result in a BadMatch error. Whether
or not given device can support this request can be determined by the information
returned by XOpenDevice. For those devices that support focus, XOpenDevice
will return an XInputClassInfo structure with the input_class field equal
to the constant FocusClass (defined in the file XI.h).

The XGetDeviceFocus
request returns the focus window and the current focus state.

Not all input
extension devices can be focused. Attempting to query the focus state of
a device that can't be focused results in a BadMatch error. A device that
can be focused returns information for input Class Focus when an XOpenDevice
request is made.

An invalid device was specified. The specified device
does not exist or has not been opened by this client via XOpenInputDevice.
This error may also occur if the specified device is the X keyboard or
X pointer device.

BadValue

Some numeric value falls outside the range of
values accepted by the request. Unless a specific range is specified for
an argument, the full range defined by the argument's type is accepted.
Any argument defined as a set of alternatives can generate this error.

BadWindow

A value for a Window argument does not name a defined Window.

BadMatch

This
error may occur if an XGetDeviceFocus or XSetDeviceFocus request was made
specifying a device that the server implementation does not allow to be
focused.